While physical variables are usually continuous signals, digital sensors can only output discrete readings of those variables. The discrete readings of such a continuous physical variable by a digital sensor may generate some noise or modify the accuracy of the digital expression of the said continuous physical variable. While filters may be applied to reduce or suppress such noise, they may then also introduce perceptible latency/delay or mask slow, but definite, variations along the series of measures of said physical variable. It is thus advantageous to develop a stabilisation method which can suppress noise in the series of measures of a physical variable captured by a digital sensor without significantly affecting such slow, but definite variations, and without introducing an additional perceptible delay.
Such a stabilisation method is particularly advantageous for stabilising a series of spatial position measurements captured by a digital sensor over time, and in particular a series of spatial position measurements captured by a 3D digital imaging system over time. In particular when the capture of such a spatial position measurement also involves an object recognition step, the object recognition algorithm may induce significant noise in successive measurements of the spatial position of the object, noise which may be aggravated by the limited definition of the spatial positioning raster.
An attempt has been made in US Patent Application publication US 2006/028944 to suppress noise in a chronological series of position measurements captured by a digital motion sensor by discarding or attenuating a change in a position measurement if the difference with a previous position measurement is below a certain threshold. However, such a stabilisation technique could lead to a slow, but real continuous motion being unnoticed or reflected as a jerking variation in the position measurements. Apparently for this reason, according to this disclosure, this technique is only applied punctually in response to a particular event.
US 2008/165280 discloses a method of video stabilisation in which the stabilisation of an image is achieved using an initial set of image stabilisation parameters in the form of a stabilisation algorithm. Both causal and non-causal filtering is described where the decoded images are buffered when causal filtering is used. A comparison of partial projection vectors, partial areas of two images, is carried out to determine the best match (the minimum distance between the two vectors being compared) for a jitter component. A filter formula is used to compensate for jitter in two-dimensions, the filter formula being expressed in terms of the computed motion estimate. A damping factor, α, is disclosed that steers the jitter towards zero and can be adapted from frame to frame to compensate for handshake of the video camera at frequencies≧2 Hz. The damping factor has a normalised value where 0<α<1.
US 2008/030460 discloses the comparison of a reference image to a comparison image to match features. Two threshold values are applied that effectively splits the image into three regions—a high damping region, a low damping region and a gradual or linear region. Positional information in the image is used to control the application, for example, the position of a hand within a region of interest.